The present invention relates to a process for the production of aliphatic and/or cycloaliphatic polyisocyanates containing allophanate groups.
Processes for the production of polyisocyanates containing allophanate groups are known. Such processes are disclosed, for example, in U.S. Pat. Nos. 4,177,342, 4,160,080 and 3,769,318 and British Pat. No. 994,890.
British Pat. No. 994,890 discloses a process for the production of polyisocyanates containing allophanate groups in which a urethane isocyanate is reacted with an excess of another diisocyanate. This reaction which may be carried out in the presence of specified catalysts is allowed to continue until the isocyanate content is reduced to that theoretically obtained when complete reaction of the urethane groups is achieved. The products obtained by this process are not however limited to the desired allophanates. Substantial amounts of dimerization and trimerization products are also present particularly where a catalyst has been used. Further, the long reaction times at elevated temperatures required in this disclosed process (at least 24 hours) where no catalyst is used result in discoloration of the final product.
This problem of preparing allophanate polyisocyanates which are not contaminated with dimeric and trimeric polyisocyanates was addressed in U.S. Pat. No. 3,769,318. According to U.S. Pat. No. 3,769,318 allophanates containing at least one aromatically bound isocyanate group are produced by reacting an N-substituted carbamic acid ester with an isocyanate in the presence of a dialkyl ester of sulfuric acid. This reaction is carried out at temperatures of from 20.degree. to 180.degree. C. for 2 to 50 hours. This processiis not, however, suitable for the preparation of allophanate polyisocyanates having aliphatically and/or cycloaliphatically bound isocyanate groups.
U.S. Pat. No. 4,160,080 discloses a process for producing allophanates containing aliphatically and/or cycloaliphatically bound isocyanate groups in which compounds containing urethane groups are reacted with polyisocyanates having aliphatic and/or cycloaliphatic isocyanate groups in the presence of a strong acid. This process is generally carried out at a temperature of from 90.degree. to 140.degree. C. for from 4 to almost 20 hours. Such reaction times are a substantial improvement over the process of British 994,890. However, discoloration of the final product due to exposure to higher temperatures for such prolonged periods is still a problem.
U.S. Pat. No. 4,177,342 discloses a process for the production of polyisocyanates containing allophanate groups in which an organic polyisocyanate is reacted with a hydroxyl carbamate compound. The allophanate is formed at a temperature of from 100.degree. to 180.degree. C. Appropriate reaction times are described as being from 20 minutes to 4 hours. However, it is clear from the examples that a temperature greater than 150.degree. C. was not actually employed. The products of the process are viscous, colorless to yellow polyisocyanates which are liquid or solid hard resins at room temperature.